1. Field of the Invention
The present invention relates to a control device for an automatic transmission including both a mechanical oil pump driven by an engine and an electric oil pump driven by an electric motor, and adapted so that working oil pressure is supplied to a transmission by performing switching to the electric oil pump from the mechanical oil pump at the time of an idle stop.
2. Description of the Background Art
Conventionally, there has been developed an engine control device adapted to stop an engine automatically when a predetermined engine automatic stop condition is satisfied at the time of traffic congestion or stopping the vehicle to wait for a signal for the purpose of saving the fuel which drives the engine, reduction of exhaust gas generated by combustion of the fuel, or the like, and thereafter to restart the engine automatically when a restart condition, such as a starting operation, is satisfied.
In such an engine control device, during an automatic stop of the engine, the mechanical oil pump driven by the engine stops. Therefore, the oil pressure for speed changing in a hydraulic control circuit included in the automatic transmission decreases, and a forward clutch will be disengaged with the hydraulic control circuit remaining at a first speed. As a result, next, when the engine has been automatically restarted, there is a problem in that the oil pressure which has abruptly risen due to the driving of the mechanical oil pump is supplied to the forward clutch as the oil pressure for engagement without being controlled, and an engagement shock is generated.
Thus, there is known a technique including an electric oil pump driven by an electric motor separately from the above mechanical oil pump driven by the engine, driving this electric oil pump during the automatic stop of the engine, thereby generating working oil pressure, and supplying this working oil pressure to a forward clutch as oil pressure for engagement, thereby maintaining a forward clutch in an engagement state where torque can be transmitted or in a state immediately before engagement.
Generally, in a case where the electric oil pump is mounted, in order to supply working oil pressure to the automatic transmission during an engine automatic stop, the electric oil pump is driven, and the power consumption of an electric storage device, such as a battery, increases. Moreover, as mentioned above, the case where the electric oil pump is driven is during an engine automatic stop. Therefore, the power generation by a power generator is also stopped, and the electric storage device is not supplied with electric power, but just consumes electric power.
That is, in order to maintain the forward clutch during an engine automatic stop in an engagement state or in a state immediately before engagement, it is desired to drive the electric oil pump so as to generate the minimum required working oil pressure, thereby suppressing the power consumption from the electric storage device as much as possible. That is, when the working oil pressure generated by the electric oil pump generates the working oil pressure which is higher than a pressure required to maintain the forward clutch in an engagement state or in a state immediately before engagement at the time of an engine automatic stop, the electric power of the electric storage device is consumed in vain. On the other hand, when the working oil pressure becomes a working oil pressure which is lower than the required working oil pressure, the forward clutch is slightly opened, and the forward clutch engages suddenly at the time of an engine restart. As a result, it becomes impossible to avoid problems, such as the generation of shock, the soaring of the number of engine rotations, or the deterioration of durability caused by a slip of the clutch.
In order to avoid these problems and to drive the electric oil pump so as to generate a minimum working oil pressure capable of maintaining the forward clutch in an engagement state or in a state immediately before engagement, it is considered that a control is performed by the feedback control of changing the driving output (driving voltage, driving duty, or the like) of the electric oil pump so that the working oil pressure comes to be within a target working oil pressure while the working oil pressure which is actually generated is detected, or that the driving output of the electric oil pump which becomes a preset target working oil pressure is obtained in advance by an experiment or the like, and the open control of driving the electric pump by the output which has been obtained in advance is performed.
Here, for example, Japanese Patent No. 3842150 suggests, in order to perform the feedback control, a technique of controlling driving of the electric oil pump not according to an automatic stop command of the engine but according to the detection value of an oil pressure sensor which detects the line pressure in the hydraulic control circuit, thereby driving or stopping the electric oil pump with suitable timing to suppress power consumption and guarantee start responsiveness.
However, in the method suggested in Japanese Patent No. 3842150, it is necessary to provide an expensive oil pressure sensor for measuring the actual oil pressure in the hydraulic control circuit for supplying the oil pressure for engagement to the forward clutch and the control operation also becomes complicated. Therefore, a cost increase will be caused in terms of both hardware and software.
On the other hand, in the case of the open control, the problem of such a cost increase does not exist. However the amount of driving of the electric oil pump for generating the target working oil pressure, i.e., the working oil pressure maintaining the forward clutch in an engagement state or in a state immediately before engagement, should be accurately obtained in advance. However, since the degree of leakage of the working oil in a valve body which constitutes the hydraulic control circuit changes individually depending on individual differences, temporal change, or the like regarding the valve body, the driving output of the electric oil pump which becomes the target working oil pressure is not uniformly determined. As a result, even if the electric oil pump is driven with the same driving output, there is a problem in that oil pressure higher than needed is generated at a certain time, and electric power is consumed in vain, or oil pressure lower than a target oil pressure is generated at a certain time, and an engagement shock or soaring occurs.
For this reason, JP-A-2006-170399 suggests the technique of measuring the driving output of the electric oil pump and the characteristics of oil pressure, not using an oil pressure sensor but using a less expensive oil pressure switch, and obtaining the driving output of the electric oil pump, thereby suppressing power consumption and guaranteeing start responsiveness. However, even in the method suggested in JP-A-2006-170399, it is necessary to provide the oil pressure switch for measuring the oil pressure characteristics, and a cost increase is caused.
Additionally, a technique of controlling the supply of oil pressure to the forward clutch according to a target number of turbine rotations without using an additional sensor, thereby maintaining the forward clutch in a state immediately before engagement is suggested in JP-A-2006-189113.
JP-A-2006-189113 is intended for a hybrid vehicle including an electric motor or the like separately from an engine, and capable of traveling even during an engine stop. In the suggested technique, the vehicle travels through a driving force other than the engine, such as an electric motor, even during an engine stop, and thereby a transmission is driven. At this time, it becomes possible to control the supply of oil pressure to the forward clutch, thereby controlling the number of turbine rotations to be a target number of rotations. That is, it is required that the vehicle is able to start and travel using other driving forces even during an engine stop. For this reason, in an idle stop vehicle in which the only source of driving force for making the vehicle travel is the engine, and which cannot travel without an engine restart when the engine stops, there is a problem in that the technique suggested in JP-A-2006-189113 cannot be executed.